The trouble is I would evaluate both side of your statement as true: I would say it's impossible to judge or (fully) understand FP+ before you use it. At best, we can make a preliminary judgement based on our current understanding of the rollout version of FP+. The first problem is your preliminary judgement could be wrong. How many times have you gone to a movie you thought you were going to like and hated it, or liked I movie you initially thought you'd hate. Second, I don't think anybody understands what's going to happen once FP+ get's fully rolled out. At best we can get an indication based on whats happening at AK.
On the other side, at this point, I'm not sure anybody can make the perfect FP+ decision 60 days out. At best, you can make an intelligent best guess based on preconceived notions of how things are going to work. I could take a couple of real trips before you start making really intelligent decisions about your FP+.
Most of this stuff can also be said about FP- over the years. It took people time to develop the FP- strategies they use today. Even at that, over the years thing could happen to make your plans go awry. Back several of years ago there were testing on RNR a version of the lineless wait system currently being used on Dumbo. There was no single rider line, no FP line, everyone got a slip of paper with a number on it. You had to wait there in the plaza area until your number came up on a big board. At that point, your group would go almost directly into the preshow. That probably put a kink in quite a few peoples FP strategy.
My next trip is a year away. I'm cautiously optimistic about FP+. Based on what we know now, I think I have an idea about how I want to book my FP+. But, as time goes on, I can see that changing.